Biomedical subjects

Azad Bonni

Publications and source records attributed to Azad Bonni.

At least 19 recordsLinked to original sources

PubMed2007

Thinking within the D box: initial identification of Cdh1-APC substrates in the nervous system.

The anaphase-promoting complex (APC) has a well-established role in cell cycle control, but recent exciting evidence has uncovered unexpected neurobiological functions for this complex E3 ubiquitin ligase. With its co-activator Cdh1, APC's effects upon the nervous system range from regulation of axon growth and patterning to development of synapses to neuronal survival. The Cdh1-APC substrates that control these biological processes in neurons are just beginning to be identified. These findings may offer a glimpse of the wide spectrum of neural activities that are orchestrated by Cdh1-APC.

Anaphase-Promoting Complex-Cyclosome↗

PubMed2006

p38 MAP kinase mediates apoptosis through phosphorylation of BimEL at Ser-65.

The stress-activated c-Jun N-terminal protein kinase (JNK) and p38 mitogen-activated protein (MAP) kinase (p38) regulate apoptosis induced by several forms of cellular insults. Potential targets for these kinases include members of the Bcl-2 family proteins, which mediate apoptosis generated through the mitochondria-initiated, intrinsic cell death pathway. Indeed, the activities of several Bcl-2 family proteins, both pro- and anti-apoptotic, are controlled by JNK phosphorylation. For example, the pro-apoptotic activity of Bim(EL), a member of the Bcl-2 family, is stimulated by JNK phosphorylation at Ser-65. In contrast, there is no reported evidence that p38-induced apoptosis is due to direct phosphorylation of Bcl-2 family proteins. Here we report evidence that sodium arsenite-induced apoptosis in PC12 cells may be due to direct phosphorylation of Bim(EL) at Ser-65 by p38. This conclusion is supported by data showing that ectopic expression of a wild type, but not a non-phosphorylatable S65A mutant of Bim(EL), potentiates sodium arsenite-induced apoptosis and by experiments showing direct phosphorylation of Bim(EL) at Ser-65 by p38 in vitro. Furthermore, sodium arsenite induced Bim(EL) phosphorylation at Ser-65, which was blocked by p38 inhibition. This study provides the first example whereby p38 induces apoptosis by phosphorylating a member of the Bcl-2 family and illustrates that phosphorylation of Bim(EL) on Ser-65 may be a common regulatory point for cell death induced by both JNK and p38 pathways.

Animals↗

PubMed2006

Degradation of Id2 by the anaphase-promoting complex couples cell cycle exit and axonal growth.

In the developing nervous system, Id2 (inhibitor of DNA binding 2, also known as inhibitor of differentiation 2) enhances cell proliferation, promotes tumour progression and inhibits the activity of neurogenic basic helix-loop-helix (bHLH) transcription factors. The anaphase promoting complex/cyclosome and its activator Cdh1 (APC/C(Cdh1)) restrains axonal growth but the targets of APC/C(Cdh1) in neurons are unknown. Id2 and other members of the Id family are very unstable proteins that are eliminated as cells enter the quiescent state, but how they are targeted for degradation has remained elusive. Here we show that Id2 interacts with the core subunits of APC/C and Cdh1 in primary neurons. APC/C(Cdh1) targets Id2 for degradation through a destruction box motif (D box) that is conserved in Id1 and Id4. Depletion of Cdh1 stabilizes Id proteins in neurons, whereas Id2 D-box mutants are impaired for Cdh1 binding and remain stable in cells that exit from the cell cycle and contain active APC/C(Cdh1). Mutants of the Id2 D box enhance axonal growth in cerebellar granule neurons in vitro and in the context of the cerebellar cortex, and overcome the myelin inhibitory signals for growth. Conversely, activation of bHLH transcription factors induces a cluster of genes with potent axonal inhibitory functions including the gene coding for the Nogo receptor, a key transducer of myelin inhibition. Degradation of Id2 in neurons permits the accumulation of the Nogo receptor, thereby linking APC/C(Cdh1) activity with bHLH target genes for the inhibition of axonal growth. These findings indicate that deregulated Id activity might be useful to reprogramme quiescent neurons into the axonal growth mode.

Anaphase-Promoting Complex-Cyclosome↗

PubMed2006

A conserved MST-FOXO signaling pathway mediates oxidative-stress responses and extends life span.

Oxidative stress influences cell survival and homeostasis, but the mechanisms underlying the biological effects of oxidative stress remain to be elucidated. Here, we demonstrate that the protein kinase MST1 mediates oxidative-stress-induced cell death in primary mammalian neurons by directly activating the FOXO transcription factors. MST1 phosphorylates FOXO proteins at a conserved site within the forkhead domain that disrupts their interaction with 14-3-3 proteins, promotes FOXO nuclear translocation, and thereby induces cell death in neurons. We also extend the MST-FOXO signaling link to nematodes. Knockdown of the C. elegans MST1 ortholog CST-1 shortens life span and accelerates tissue aging, while overexpression of cst-1 promotes life span and delays aging. The cst-1-induced life-span extension occurs in a daf-16-dependent manner. The identification of the FOXO transcription factors as major and evolutionarily conserved targets of MST1 suggests that MST kinases play important roles in diverse biological processes including cellular responses to oxidative stress and longevity.

14-3-3 Proteins↗

PubMed2006

Cell-intrinsic regulation of axonal morphogenesis by the Cdh1-APC target SnoN.

Axonal growth is fundamental to the establishment of neuronal connectivity in the brain. However, the cell-intrinsic mechanisms that govern axonal morphogenesis remain to be elucidated. The ubiquitin ligase Cdh1-anaphase-promoting complex (Cdh1-APC) suppresses the growth of axons in postmitotic neurons. Here, we report that Cdh1-APC operates in the nucleus to inhibit axonal growth. We also identify the transcriptional corepressor SnoN as a key target of neuronal Cdh1-APC that promotes axonal growth. Cdh1 forms a physical complex with SnoN and stimulates the ubiquitin-dependent proteasomal degradation of SnoN in neurons. Knockdown of SnoN in neurons significantly reduces axonal growth and suppresses Cdh1 RNAi enhancement of axonal growth. In addition, SnoN knockdown in vivo suggests an essential function for SnoN in the development of granule neuron parallel fibers in the cerebellar cortex. These findings define Cdh1-APC and SnoN as components of a cell-intrinsic pathway that orchestrates axonal morphogenesis in a transcription-dependent manner in the mammalian brain.

Anaphase-Promoting Complex-Cyclosome↗

PubMed2006

Pin1 mediates neural-specific activation of the mitochondrial apoptotic machinery.

Apoptosis of neurons plays fundamental roles in brain development and disease. Although neurons share with other cell types components of the mitochondrial apoptotic machinery, how this machinery is specifically activated in neurons remains poorly understood. Remarkably, phosphorylation of the BH3-only protein BIMEL at Ser65 triggers apoptosis in neurons but suppresses cell death in non-neural cells. Here, we report that the prolyl isomerase Pin1 interacts with Ser65-phosphorylated BIMEL in neurons. Pin1 is enriched at the mitochondrial membrane in neurons, where it forms a physical complex with the neuron-specific JNK scaffold protein JIP3. Activation of JNK signaling induces the dissociation of Pin1 from JIP3 and concomitantly promotes Pin1 binding to phosphorylated BIMEL. The interaction of Pin1 with phosphorylated BIMEL stabilizes BIMEL and thereby activates neuronal apoptosis. These findings define a neural-specific mechanism of cell death whereby Pin1 couples phosphorylation of BH3-only proteins to activation of the mitochondrial apoptotic machinery.

Adaptor Proteins, Signal Transducing↗

PubMed2006

A calcium-regulated MEF2 sumoylation switch controls postsynaptic differentiation.

Postsynaptic differentiation of dendrites is an essential step in synapse formation. We report here a requirement for the transcription factor myocyte enhancer factor 2A (MEF2A) in the morphogenesis of postsynaptic granule neuron dendritic claws in the cerebellar cortex. A transcriptional repressor form of MEF2A that is sumoylated at lysine-403 promoted dendritic claw differentiation. Activity-dependent calcium signaling induced a calcineurin-mediated dephosphorylation of MEF2A at serine-408 and, thereby, promoted a switch from sumoylation to acetylation at lysine-403, which led to inhibition of dendritic claw differentiation. Our findings define a mechanism underlying postsynaptic differentiation that may modulate activity-dependent synapse development and plasticity in the brain.

Acetylation↗

PubMed2006

Modeling oxidative stress in the central nervous system.

Oxidative stress is associated with the onset and pathogenesis of several prominent central nervous system disorders. Consequently, there is a pressing need for experimental methods for studying neuronal responses to oxidative stress. A number of techniques for modeling oxidative stress have been developed, including the use of inhibitors of the mitochondrial respiratory chain, depletion of endogenous antioxidants, application of products of lipid peroxidation, use of heavy metals, and models of ischemic brain injury. These experimental approaches can be applied from cell culture to in vivo animal models. Their use has provided insight into the molecular underpinnings of oxidative stress responses in the nervous system, including cell recovery and cell death. Reactive oxygen species contribute to conformational change-induced activation of signaling pathways, inactivation of enzymes through modification of catalytic cysteine residues, and subcellular redistribution of signaling molecules. In this review, we will discuss several methods for inducing oxidative stress in the nervous system and explore newly emerging concepts in oxidative stress signaling.

Central Nervous System↗

PubMed2005

Moving past proliferation: new roles for Cdh1-APC in postmitotic neurons.

Nearly ten years after its discovery as a crucial cell-cycle-regulated ubiquitin ligase, the anaphase-promoting complex (APC) is making a debut in neurobiology. During the past year, some of the mystery surrounding a potential function for APC in the brain has been unveiled. Recent studies have defined novel roles for APC in the regulation of axonal growth and patterning, as well as in synaptic development and function. With this strong beginning for APC in neurobiology, the months and years to come are likely to bring many more insights into how neuronal APC contributes to biological processes in brain development and disease.

Anaphase-Promoting Complex-Cyclosome↗

PubMed2005

Phosphorylation of BAD at Ser-128 during mitosis and paclitaxel-induced apoptosis.

Phosphorylation of BCL-2 family member BAD at different residues triggers different physiological effects, either inhibiting or promoting apoptosis. The recently identified phosphorylation site at Ser-128 enhances the apoptotic activity of BAD. We here show that BAD becomes phosphorylated at Ser-128 in the mitotic phase of the cell cycle in NIH3T3 cells. We also show that BAD-S128 is phosphorylated in taxol-treated mouse fibroblasts and MDA-MB-231 human breast cancer cells. However, expression of a phosphorylation-defective dominant negative BAD mutant did not block taxol-induced apoptosis. These data support the view that the phosphorylation of BAD Serine 128 exerts cell-specific effects on apoptosis. Whereas the BAD Serine 128 phosphorylation induces apoptosis in neuronal cells, it does not appear to promote apoptosis in proliferating non-neural cells during mitosis or upon exposure to the antineoplastic agent taxol.

Animals↗

PubMed2005

Beyond proliferation--cell cycle control of neuronal survival and differentiation in the developing mammalian brain.

Cell cycle proteins are critical regulators of proliferation in dividing cells. Paradoxically, accumulating evidence supports the view that core components of the cell cycle also play key roles in the development of terminally differentiated postmitotic neurons. Distinct cell cycle proteins including cell cycle-dependent kinases may contribute to naturally occurring programmed neuronal cell death in the developing mammalian brain. In addition, recent studies have uncovered a novel role for the cell cycle-associated ubiquitination machinery in the control of axonal growth and patterning in the developing brain. The underlying molecular mechanisms regulating these distinct cell cycle-based developmental events in neurons are just beginning to be understood.

Animals↗

PubMed2005

brawn for brains: the role of MEF2 proteins in the developing nervous system.

The myocyte enhancer factor 2 (MEF2) transcription factors were originally identified, as their family name implies, on the basis of their role in muscle differentiation. Expression of the four MEF2 proteins, however, is not restricted to contractile tissue. While it has been known for more than a decade that MEF2s are abundantly expressed in neurons, their contributions to the development and function of the nervous system are only now being elucidated. Interestingly, the emerging mechanisms regulating MEF2 in neurons have significant parallels with the regulatory mechanisms in muscle, despite the quite distinct identities of these two electrically excitable tissues. The goal of this chapter is to provide an introduction to those regulatory mechanisms and their consequences for brain development. As such, we first provide an overview of MEF2 itself and its expression within the central nervous system. The second part of this chapter describes the signaling molecules that regulate MEF2 transcriptional activity and their contributions to MEF2 function. The third part of this chapter discusses the role of MEF2 proteins in the developing nervous system and compares the analogous functions of this protein family in muscle and brain.

Animals↗

PubMed2005

Bim regulation of lumen formation in cultured mammary epithelial acini is targeted by oncogenes.

Epithelial cells organize into cyst-like structures that contain a spherical monolayer of cells that enclose a central lumen. Using a three-dimensional basement membrane culture model in which mammary epithelial cells form hollow, acinus-like structures, we previously demonstrated that lumen formation is achieved, in part, through apoptosis of centrally localized cells. We demonstrate that the proapoptotic protein Bim may selectively trigger apoptosis of the centrally localized acinar cells, leading to temporally controlled lumen formation. Bim is not detectable during early stages of three-dimensional mammary acinar morphogenesis and is then highly upregulated in all cells of acini, coincident with detection of apoptosis in the centrally localized acinar cells. Inhibition of Bim expression by RNA interference transiently blocks luminal apoptosis and delays lumen formation. Oncogenes that induce acinar luminal filling, such as ErbB2 and v-Src, suppress expression of Bim through a pathway dependent on Erk-mitogen-activated protein kinase; however, HPV 16 E7, an oncogene that stimulates cell proliferation but not luminal filling, is unable to reduce Bim expression. Thus, Bim is a critical regulator of luminal apoptosis during mammary acinar morphogenesis in vitro and may be an important target of oncogenes that disrupt glandular epithelial architecture.

Adaptor Proteins, Signal Transducing↗

PubMed2004

The transcription factor NFAT3 mediates neuronal survival.

Neuronal apoptosis is critical for normal development of the mammalian nervous system and also contributes to the pathogenesis of ischemic and degenerative diseases of the brain. Apoptosis of neurons is tightly regulated by extrinsic signals including growth factors and neuronal activity, but the intracellular mechanisms by which these signals promote neuronal survival are incompletely understood. We report that the transcription factor NFAT3 plays a critical role in mediating survival of granule neurons of the developing cerebellum. NFAT3 accumulated in the nucleus of primary granule neurons under survival conditions of serum growth factors and neuronal activity that was elicited by depolarization with high K(+). In contrast, deprivation of serum and K(+), which leads to neuronal apoptosis, triggered NFAT3 nuclear export. Treatment of granule neurons with Li(+), an inhibitor of the NFAT export kinase GSK3, prevented the nuclear export of NFAT3 and increased granule cell survival even under pro-apoptotic conditions. Thus, the nuclear localization of NFAT3 correlated tightly with granule neuron survival. Consistent with a pro-survival function for NFAT3, genetic knockdown of NFAT3 by RNA interference in primary granule neurons led to increased apoptosis even in neurons cultured under survival conditions. Conversely, expression of a constitutively active form of NFAT protected neurons against apoptosis induced by serum withdrawal and low K(+). Taken together, these results reveal an essential function for NFAT3-mediated transcription in neuronal survival that may play important roles in the developing and mature brain.

Active Transport, Cell Nucleus↗

PubMed2004

Characterization of the c-Jun N-terminal kinase-BimEL signaling pathway in neuronal apoptosis.

The c-Jun N-terminal kinase (JNK) signaling pathway plays a critical role in mediating apoptosis in the nervous system; however, the mechanisms by which JNK triggers neuronal apoptosis remain incompletely understood. Recent studies suggest that in addition to inducing transcription of pro-apoptotic genes, JNK also directly activates the cell death machinery. Here, we report that JNK catalyzed the phosphorylation of the BH3-only protein Bcl-2 interacting mediator of cell death (BimEL) at serine 65, both in vitro and in vivo. The JNK-induced phosphorylation of BimEL at serine 65 promoted the apoptotic effect of BimEL in primary cerebellar granule neurons. We also characterized the role of the JNK-BimEL signaling pathway in apoptosis that was triggered by overexpression of the p75 neurotrophin receptor (p75NTR). We found that activation of p75NTR induced the JNK-dependent phosphorylation of endogenous BimEL at serine 65 in cells. The genetic knockdown of BimEL by RNA interference or the expression of a dominant interfering form of BimEL significantly impaired the ability of activated p75NTR to induce apoptosis. Together, these results suggest that JNK-induced phosphorylation of BimEL at serine 65 mediates p75NTR-induced apoptosis. Our findings define a novel mechanism by which a death-receptor pathway directly activates the mitochondrial apoptotic machinery.

Amino Acid Sequence↗

PubMed2004

A CaMKII-NeuroD signaling pathway specifies dendritic morphogenesis.

The elaboration of dendrites is fundamental to the establishment of neuronal polarity and connectivity, but the mechanisms that underlie dendritic morphogenesis are poorly understood. We found that the genetic knockdown of the transcription factor NeuroD in primary granule neurons including in organotypic cerebellar slices profoundly impaired the generation and maintenance of dendrites while sparing the development of axons. We also found that NeuroD mediated neuronal activity-dependent dendritogenesis. The activity-induced protein kinase CaMKII catalyzed the phosphorylation of NeuroD at distinct sites, including endogenous NeuroD at Ser336 in primary neurons, and thereby stimulated dendritic growth. These findings uncover an essential function for NeuroD in granule neuron dendritic morphogenesis. Our study also defines the CaMKII-NeuroD signaling pathway as a novel mechanism underlying activity-regulated dendritic growth that may play important roles in the developing and mature brain.

Animals↗

PubMed2004

Cdh1-APC controls axonal growth and patterning in the mammalian brain.

The anaphase-promoting complex (APC) is highly expressed in postmitotic neurons, but its function in the nervous system was previously unknown. We report that the inhibition of Cdh1-APC in primary neurons specifically enhanced axonal growth. Cdh1 knockdown in cerebellar slice overlay assays and in the developing rat cerebellum in vivo revealed cell-autonomous abnormalities in layer-specific growth of granule neuron axons and parallel fiber patterning. Cdh1 RNA interference in neurons was also found to override the inhibitory influence of myelin on axonal growth. Thus, Cdh1-APC appears to play a role in regulating axonal growth and patterning in the developing brain that may also limit the growth of injured axons in the adult brain.

Anaphase-Promoting Complex-Cyclosome↗

PubMed2004

Cell cycle regulation of neuronal apoptosis in development and disease.

Apoptosis of neurons is indispensable to the normal development of the nervous system and contributes to neuronal loss in neurologic injury and disease. Life and death decisions are imposed upon neurons by extracellular and intracellular stimuli including the lack of trophic support, exposure to neurotoxins, oxidative stress, and DNA damage. These stimuli induce signaling pathways that are integrated at the mitochondrial apoptotic machinery culminating in cell survival or death. Growing evidence suggests that cell cycle proteins are expressed in dying neurons in the developing and adult brain. However, the role and mechanisms by which re-activation of cell cycle pathways in postmitotic neurons propagates an apoptotic signal to the cell death machinery are just beginning to be characterized. Here, we will review the molecular mechanisms of neuronal cell death and survival with a focus on recent findings on cell cycle regulation of neuronal apoptosis in primary cultures of neurons, mouse models of neuronal diseases, and human neurodegenerative diseases.

Animals↗